The present invention relates to control circuits for speakerphone circuits.
A typical speakerphone circuit has a transmit channel coupled between a microphone and the telephone line, and a receive channel coupled between the telephone line and a speaker. Each of the channels has an attenuator or amplifier whose gain can be adjusted by a control circuit. In a typical "four-point" sensing scheme, the signal before and after the attenuator in the transmit and receive channels are monitored, providing four inputs to a control circuit. In conjunction with other signals, the control circuit uses the four-point sense inputs to switch the speakerphone to the channel with the greater speech signal.
Where voice signals are detected in both channels, one scheme adjusts the gain of the two amplifiers so they are equal, and then measures the two signals to determine which is larger, switching to the channel which has the larger signal. This allows one party to break in on the other, even though the amplifier for the interrupting party is attenuated compared to the other channel. Such a system is shown, for instance, in U. S. Pat. No. 4,724,540. This mode where both gains are equal is referred to as an "idle mode". Thus, a two-step process is followed to allow transition to another channel. First, the gains are equalized to enter into the "idle mode". Second, the amplitudes of the signals are compared while in the idle mode, and the channel with the highest amplitude signal is provided with the greater gain, to activate that channel.
Only one channel at a time can be provided with high gain in order to avoid "singing", caused by signal from the speaker feeding back into the microphone through acoustic coupling or by signal from the microphone feeding back through the hybrid side tone to the speaker through electrical coupling.
U. S. Pat. No. 4,879,745, assigned to IBM, shows a speakerphone in which the Signals are converted into digital form. That digital speakerphone then uses a synchronous state machine to control the selection of either the transmit or receive audio path. Such a system provides the advantage of more flexibility in switching paths, but at the cost of the additional complexity of the digital circuitry.